From this chapter, it sounds like Kotone and Sora had something a long time ago, but it is also possible that he was the one she was engaged to. It would explain her sudden addition to the plot. The problem is that a wealthy family marrying off its children for status wouldn't accept same-sex marriages anyway, since they don't exist in any real form in Japan right now (the civil partnerships offered by a few places there pale in comparison to actual marriage rights).